That would be a sensible, albeit modest, proposal from a mindflayer emissary. "We understand your concern about us feeding over your most intelligent and productive society members. As a gesture of good-will, we propose that you send us 10 mentally deficient members of your race, that would be a burden on your primitive societies, in order for us to feed. It's bland and frankly, but we are willing to seek compromise as we're, obviously, extremely enlightened, especially compared to you. Or maybe you'd be OK if we went and ate the elves in the nearby wood?" Instant horror, credible motivation and a way for your heroes to understand that their opponents are not evil because of their diet. The onus to find an acceptable counter-proposal would be on them, or accept that, despite having a reasonable opponent not necessarily evil -- he was seeking to compromise after all -- his vital goals are incompatible with ours and therefore they
must die as a species, without having to decide if any particular one is good or evil.
That.
The squid and cephalopod community shakes with horror as they're forced in the kitchen to watch their lobster friend being boiled... ALIVE !
Bonus points if the whole human family at the restaurant gathered around the fish tank to select which lobster they'll have cooked for them.
My test is quicker. If you ascribe generic qualities to all representatives of a gender, it's sexist, if you ascribe qualities to all representatives of a country, it's xenophobic, if you ascribe qualities to all members of a race, it's racist. They don't necessarily have to be "bad" qualities. "Women are good with children" is positive, yet sexist. "Indians are good with computers" or "Asians are hard-working and respectful of elder people" is xenophobic or racist, respectively. The concept of judging an individual based not on him, but on qualities ascribed to a group he's a member of is deeply problematic and the underlying mode of thought of racism/xenophobia. If you do that with orcs or mind flayers or whatever, you're emulating the racist mindset. You should first make sure that the creature in front of you is either not sentient and unable to make other choices (so you wouldn't need to discuss or redeem a xenomorph or a covid-19 strain) or actually guilty of the evil you're accusing them of. Or accept that you're killing them because it's fun and convenient (we generally do things in entertainment that we wouldn't endorse in real life and that doesn't have a weight on our morality).